The invention is related generally to antennas, and more particularly, to multi-frequency band antennas capable of dual polarization operation.
Compact monopulse antennas are useful in many applications and a dual polarization antenna usable for monopulse operation is also desirable for many applications. An ideal antenna with relatively wide angular coverage, such as a 3 dB beamwidth at 90.degree., circularly polarized with rotationally symmetric radiation patterns, could, in principle, consist of a circular current loop with current distribution e.sup..+-.jm.phi., where .phi. is the azimuthal angle and m is an integer. The m=1 mode radiates a circularly polarized wave with maximum normal to the plane of the loop and has a rotationally symmetric radiation pattern. The m=0 and m=even modes have radiation patterns with a null on the axis normal to the loop. The m=2 mode is most often used to generate monopulse "difference" patterns.
Isolated current loop type antennas have bi-directional radiation; i.e., they radiate equally in both hemispheres on the axis normal to the plane of the loop. This radiation pattern behind the antenna is undesirable in many applications, including monopulse applications. One commonly used method for eliminating this rear pattern is to couple an absorbing cavity to the back of the antenna which then absorbs energy in the rear pattern. However, such cavities can result in a significant increase in cost and weight. Another technique is to use a power absorbing ground plane to dissipate the rear pattern. While these techniques may achieve elimination of the rear pattern, they result in the loss of approximately 3 dB of antenna gain. Additionally, lossy cavities are in many cases difficult to design and are a major contributor to the cost of the antenna.
Another technique used for rear pattern elimination is the coupling of a metallic ground plane to the antenna which is placed approximately one-quarter wavelength from the plane of the antenna current loop. This technique can result in a unidirectional radiation pattern; however, the ground plane spacing is typically accurate for only one frequency band and results in reduced performance at other frequency bands.
Conical spiral antennas have been designed which result in unidirectional patterns; however, the phase center of radiation is typically not fixed but varies with frequency. Hence this design is not an efficient feed for reflectors or lenses and will usually occupy a much larger volume because of the length of the sharp cone necessary for unidirectional radiation.
"Tightly wound" archimedes or log periodic (equi-angular) spiral antennas can approximate an ideal current loop. Spirals of single and multiple arms exist. The active radiating region is equivalent to a current loop of an integral number of wavelengths in circumference. The wavelength referred to is the wavelength of the wave travelling along the spiral conductor and it is usually slightly less than the free space wavelength. Multi-arm spirals are used for monopulse operation because the "sum" and "difference" modes can be controlled by the feed network to the multiple arms.
One disadvantage of spiral antennas is the requirement for a lossy ground plane or rear absorbing cavity which results in a 3 dB reduction of antenna gain. Additionally, prior spiral antennas have only one sense of circular polarization which is determined by the sense of the spiral winding. Attempts have been made to feed the spiral from the outside to achieve circular polarization of the opposite sense. This has only been partially successful because the active region for these modes is in the outer regions of the spiral and feeding it in this region results in undesirable higher order modes.
A sinuous antenna has been disclosed which claims to be capable of radiating both senses of circular polarization equally well, see U.S. Pat. No. 4,658,262 to DuHamel. The antenna may also be able to radiate orthogonal linear polarizations. However, the antenna has a lossy cavity back and therefore loses approximately 3 dB of its gain.
Hence, those concerned with the development and use of antennas have long recognized the need for a dual polarization, unidirectional antenna which does not sacrifice 3 dB of antenna gain in a rear pattern yet is capable of a monopulse operation. The present invention fulfills these needs.